The Primary Goal of Project 3 is to establish more effective treatment of anemia in critically ill infants, who receive multiple red blood cell (RBC) transfusions and who, as a result, face increased risk of infections and other complications. We propose to reduce the number of transfusions by optimizing the use of erythropoietin (r-HuEPO). The extent to which this can be accomplished will be determined using pharmacokinetic (PK) and pharmacodynamic (PD) data from anemic, neonatal infants undergoing RBC transfusions supplemented with data from PK/PD experiments in neonatal sheep. Our overall hypothesis is that treatment of neonatal anemia can be improved by optimizing administration of r- HuEPO through a comprehensive knowledge of the physiology of erythropoiesis and EPO's complex PK/PD behavior. Aim 1 is to identify the PK/PD and physiologic factors important in predicting and maximizing r-HuEPO's erythropoietic effect based on PK/PD studies of anemic infants and neonatal sheep. Aim 2 is to make Bayesian population PK/PD efficacy predictions of r-HuEPO in anemic infants by applying allometric and physiologic scaling and a physiologic-mechanistic PK/PD model developed from the data generated in Aim 1. Aim 3 is to predict the extent to which r-HuEPO's effect (i.e. reduction/elimination of RBC transfusions) can be maximized through an optimal dosing design based on Bayesian Markov Chain Monte Carlo simulations of human clinical trials using the PK/PD model developed in Aim 2. Aim 4 is to validate a safe, non-radioactive r-HuEPO tracer (BioEPO) for use in mechanistic PK/PD studies in sheep and humans. Important physiologic and pharmacologic issues related to neonatal anemia that cannot be elucidated by current methods can be resolved if a safe and effective BioEPO tracer is developed. Project 3 will test 4 hypotheses: 1) Endogenous EPO production rate and plasma level vs. time profiles relate in a predictable manner to changes in hemoglobin vs. time profiles;2) EPO receptor (EPOR) pool size changes in a predictable way with changes in the degree of anemia and with the degree of exposure to r-HuEPO and EPO;3) Stress erythropoiesis results in a reduction in the reticulocyte mean residence time in peripheral blood due to reticulocytes being released from the bone marrow in a more mature form;4) Using an optimized r-HuEPO dosing regimen it is possible to significantly reduce the number of RBC transfusions in lamb experiments designed to mimic transfusion practices in neonatal anemia. We intend to achieve our Primary Goal by a mechanism and model-based approach assisted by powerful tracer methodologies, and through the use of advanced, modern drug development tools such as Bayesian analysis and clinical trial simulations. Our PK/PD analysis will extend the latest developments in receptor-mediated PK/PD analysis and will consider processes taking place on the cellular and receptor level, including anemia-dependent changes that affect the life-span of reticulocytes and RBCs during stress erythropoiesis. We will make use of advanced 125-l-r-HuEPO tracer kinetic methodologies to address Aim 1 and enable elucidation of the factors that impact the efficacy and utility of r-HuEPO for achieving the Primary Goal. Project 3 will interact synergistically with Project 1 and the Core Facility in several key areas of mutual interest, e.g. investigation of RBC volume and RBC life-span and mathematical modeling of the survival of biotinylated RBCs. The Primary Goal in this project and the collaborative work Dr. Veng-Pedersen will provide on Project 1 follow the current trend in clinical practice to reduce the number of neonatal transfusions and support our PPG's theme of optimizing the transfusion management of anemic newborn infants.